The present invention relates to an overcurrent protection circuit and, in particular, relates to an overcurrent protection circuit having an automatic restoring function of automatically restoring to a normal state from a power interruption state caused by an overcurrent.
In recent years, various kinds of electrical equipments driven by a vehicle-mounted battery are mounted on the vehicle. An overcurrent protection circuit is known which has a function of interrupting power supply to the electrical equipments based on the detection of abnormal exothermic heat caused by an overcurrent flowing through the electrical equipment and an automatic restoring function of automatically restoring to a normal state upon extinction of the abnormal exothermic heat. Such an interruption and automatic restoring of the power supply is performed by controlling the relay including a coil so as to be opened and closed. A conventional example is arranged in a manner that a current same as that flowing through the coil also flows through a temperature detection element for detecting the abnormal exothermic heat, so that there arise a problem that it is difficult to adjust the time for the power interruption and the time for the automatic restoration. The conventional example having such a problem will be explained with reference to FIG. 4.
FIG. 4 is a circuit diagram showing an example of a conventional overcurrent protection circuit. In this case, the explanation will be made on the assumption that the overcurrent protection circuit is incorporated within an electric coupling box of a function circuit embedded type called a junction block which is frequently employed in vehicles recently.
An electric coupling box 9 shown in FIG. 4 is mounted on the vehicle. A battery 2 and a motor 3 are coupled to the power input terminal 91 and the external output terminal 92 of the electric coupling box, respectively. The battery 2 is a known battery of 12 V and the motor 3 is a motor for driving a fan for cooling an engine, for example. The motor 3 is provided with two power source terminals 3a, 3c and two earth terminals 3b, 3d. The motor is a known four-terminal type motor which rotation speed is varied in accordance with a driving signal supplied to the power source terminals 3a, 3c. For example, when the power is supplied to one of the power source terminals 3a and 3c, the motor rotates at a low speed, whilst when the power is supplied to both the power source terminals 3a and 3c, the motor rotates at a high speed. In this case, the explanation will be made as to the case where the power is supplied to both the power source terminals 3a and 3c in order to simplify the explanation.
Within the electric coupling box 9, various kinds of electronic parts and resin parts are housed and a bus bar of a conductive plate shape for distributing the power supplied from the battery 2 is disposed. A PTC (positive temperature coefficient thermistor) 93 serving as a temperature detecting element is disposed in the vicinity of the bus bar so that an over current flowing through the bus bar is detected based on the temperature monitored by using the PTC 93. Within the electric coupling box 9, a relay 97 is provided which is arranged in a manner that a coil 97a excited by a predetermined current closes/opens a contact 97b thereby to control the power supply from the battery 2 to the motor 3. Within the electric coupling box 9, a CPU 95 is provided which is arranged to receive an external control signal inputted from a signal input terminal 94 and output a switching control signal for performing the opening/closing control of the relay 97.
In the aforesaid configuration, for example, when a not-shown ignition switch is turned on, the external control signal is inputted into the CPU 95 through the signal input terminal 94. In response to the external control signal, the CPU 95 outputs a high-level signal to turn on a transistor element 96. Simultaneously, a current flows into the coil 97a of the relay 97 from the battery 2 through the power input terminal 91 thereby to close the contact point 97b. As a result, the current from the battery 2 is supplied to the power source terminals 3a, 3c of the motor 3 through the contact point 97b of the relay 97 and the external output terminal 92, whereby the motor 3 is placed in a driven state.
In the driven state, supposing that the vehicle runs on a flooded road, for example, the fan driven by the motor 3 is locked or placed in a similar state due to the flooding. However, since the motor 3 continues to drive the fan against the resistance of the water, an overcurrent called as a lock current flows. That is, the overcurrent flows into the bus bar, and so abnormal exothermic heat is generated from the bus bar. Due to the abnormal exothermic heat, the PTC 93 abruptly increases its resistance value, so that current stops flowing into the coil 97a of the relay 97. As a result, the contact point 97b of the relay 97 opens thereby to stop the current supply to the motor 3. Thus the overcurrent disappears and so the temperature of the bus bar reduces, whereby the electric parts, the resin parts and the motor 3 are protected from the overcurrent.
On the other hand, if the current supply to the motor 3 is kept to be stopped, the cooling effect of the engine can not be obtained. Thus, in the case where the vehicle passed the flooded road and the temperature of the bus bar reduced sufficiently, for example, the motor 3 is started to be driven again due to the function reverse to the aforesaid function. That is, when the current supply is stopped and the temperature of the bus bar reduced sufficiently, the resistance value of the PTC 93 reduces and so the current starts flowing again through the coil 97a of the relay 97. As a result, the contact point 97b of the relay 97 is closed and so the current is started to be supplied to the motor 3 again thereby to drive the motor 3 again. In this manner, the conventional example has an automatic restoring function. Incidentally, the conventional example is supposed that the sufficient countermeasure for water-proof is performed.
However, since the shapes and the kinds of the bus bars, the kinds of external electric equipments etc. each being controlled in its on/off operation are not always same, it is necessary to select the PTC 93 having temperature characteristics suitable thereto. However, according to the aforesaid conventional example, since the PTC 93 and the coil 97a are connected in series, a current equal to that flowing through the coil 97a also flows through the PTC 93. Thus, in the conventional example, in order to adjust the times or timings of the power interruption and the automatic restoring in accordance with the shapes and the kinds of the bus bars and the kinds of external electric equipments etc., it is required to select the PTC 93 in accordance with the current flowing through the coil 97a. That is, the selection of the PTC 93 depends on the current flowing through the coil 97a. Thus, according to the conventional example, it is difficult to adjust the times of the power interruption and the automatic restoring as desired. In particular, since it is necessary to obtain the PTC 93 with a relatively large capacity in order to select the PTC 93 through which the current equal to that flowing through the coil 97a flows, most of the overcurrent protection circuits can not satisfy the restriction of the space within the electric coupling box which tends to be high-density.